The present invention relates to a system and method for speed and power control of an engine, and in particular, to controlling the speed and power of the engine by modulation of the load torque. The invention is useful for a variety of engines, including a Stirling cycle engine.
A Stirling cycle engine is a heat engine in which a variable volume chamber of a higher temperature is connected to a variable volume chamber of a lower temperature through a regenerator. A gaseous fluid is transferred back and forth between the chambers and through the regenerator in a closed cycle. A Stirling cycle engine may be powered by a variety of heat sources, including solar and the combustion of fossil fuels. The output mechanical energy of the engine can be used to do direct work or for the generation of electrical energy, etc.
Due to the high efficiency and low emissions of a Stirling cycle engine as compared to Otto cycle (spark ignition) and Diesel cycle internal combustion engines, Stirling engines are being looked at for use in motor vehicles to improve fuel efficiency and to reduce exhaust emissions. At present, one especially promising application of a Stirling engine in a motor vehicle is in what is called a series hybrid electric vehicle. In a series hybrid electric vehicle, the engine drives a generator to produce electricity to augment electric power stored in batteries. Electric power from the generator and batteries drive the vehicle through electric motors coupled to the traction wheels. Energy produced by the engine-generator in excess of the road demand during vehicle operation is stored in batteries and used during periods of high power demand. This enables the engine to operate continuously at a fixed engine output torque.
The assignee of the present application, Stirling Thermal Motors, Inc., has made significant advances in the technology of Stirling engines over a number of years. Examples of such innovations include the development of a compact and efficient basic Stirling engine configuration employing a parallel cluster of double acting cylinders which are coupled mechanically through a rotating swashplate. In many applications, a swashplate actuator is implemented to enable the swashplate angle and, therefore, the piston stroke to be changed in accordance with operating requirements. By changing the swashplate angle, the torque, and thus the power output of the engine, can be changed. However, to provide a variable angle swashplate adds significant complexity and cost to the Stirling engine. U.S. Pat. No. 4,994,004, herein incorporated by reference, illustrates a Stirling engine having a variable angle swashplate.
The engine torque can also be varied by changing the temperature difference between the hot and cold sides of the engine. Generally speaking, however, a Stirling engine operates most efficiently with a large temperature difference. Consequently, if torque and power are modulated by decreasing the temperature difference, the engine efficiency would also be reduced.
The working fluid mean pressure can also be varied to change engine torque and power by varying the amount of working gas in the engine. To provide a means to change the fluid pressure, however, significantly adds to the complexity and cost of the engine. Moreover, like changing temperature, such an approach to output modulation would impose an efficiency penalty.
The lowest cost and simplest engine design is one with a fixed swash plate and a constant working fluid charge. The greatest efficiency is produced with a constant maximum operating temperature. With the maximum temperature, fluid mean pressure and swashplate angle fixed, it is an objective of the present invention to provide a method of controlling engine power.
With the internal operating parameters, i.e., temperature, charge pressure and displacement of the Stirling engine fixed at chosen values, the engine develops an essentially constant torque, T.sub.E, regardless of engine speed. The engine power, P.sub.E, then is essentially proportional to the engine speed, .omega.. Speed, and thus power, may be varied by adjusting the load driven by the engine. The engine and the driven equipment (generator) will accelerate or decelerate accordingly as the load torque is decreased or increased, relative to the essentially fixed or constant engine torque. As the engine accelerates or decelerates, its power, P.sub.E, increases or decreases in proportion. The load power, P.sub.L, however, depends on the load torque, T.sub.L, since power is a product of the torque and speed. In accordance with this invention, load torque T.sub.L is manipulated in order to cause the engine speed, and hence power, P.sub.E to be changed. In order to accomplish such control, the load torque, T.sub.L must undergo a temporary excursion in the direction opposite the desired change. The temporary excursion of power in the "wrong direction" requires power from a secondary source, such as the traction batteries in hybrid vehicle applications, to assist during the transient event to smooth out the net powered delivered.
The load control approach in accordance with this invention can be used directly as part of an automatic feedback control of the engine speed. It can also be used, indirectly, as part of an automatic feedback control of the power.
Further objects, features and advantages of the invention will become apparent from a consideration of the following description and the appended claims when taken in connection with the accompanying drawings.